This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. A. Specific Aims The goal of this proposal is to determine the functional significance of prohibitins during 3T3-L1 adipocyte differentiation. Our preliminary data showed that both of these prohibitins (PHBs), including PHB1 and PHB2 (repressor of estrogen receptor activity, REA), were significantly increased during the adipocyte differentiation of 3T3-L1 preadipocytes. Moreover, 3T3-L1 adipocyte differentiation was impaired after the simultaneous knockdown of either PHBs protein by siRNA-PHB1 or siRNA-REA. So, our first aim is to examine the functional mechanism (phosphorylation and subcellular translocation) of PHBs during adipogenesis. Based on the bioinformatics analyses, we found several putative Egr-1 transcription factor binding sites in the mouse and human PHBs promoters. In addition, we observed that Egr-1 expression was induced prior to the increase of PHBs during 3T3-L1 adipocyte differentiation. Furthermore, the expression of PHBs proteins was upregulated in the adipose tissue in adipose-transgenic Egr-1 mice. Our second aim is to determine that Egr-1 is a critical modulator of PHBs upregulation during 3T3-L1 adipocyte differentiation.